This invention relates to a metallic conveyor belt arrangement for transporting bulk material. The arrangement comprises a metallic conveyor belt closed in itself and extending along a longitudinal axis. The metallic conveyor belt forms at least two ends, such as a slack end and a tight end, and at least a first and a second deflector loop connecting said ends. The metallic conveyor belt has first and second surfaces, each on one side thereof, of which one surface serves for loading the bulk material to be conveyed on it. A tensioning device for said metallic conveyor belt is arranged in the range of a second loop.
In general the metallic conveyor belt arrangement will use a belt of steel, although other metals shall not be excluded, because there are similar problems. If, in the framework of the present description xe2x80x9cbulk materialxe2x80x9d is mentioned, this should be understood in the broadest sense, for example encompassing quantities of baked goods in lumps.
Such metallic conveyor belt arrangement have to meet a series of requirements. On the one hand, they have to be easily cleaned, in order to meet hygienic regulations. On the other hand, they must be reliable in operation and have a long life. In order to meet the first requirement, within the region of the circumference of the drive wheel or the tensioning wheel at least part of that surface of steel belts on the market and according to the prior art, which is mostly formed by the inner or lower surface (if the belt was not deviated in an upward direction) and which is engaged by some parts of the drive, thus being averted from the surface where material is to be loaded, is free from engagement with the wheel to counteract against any transfer of fatty or sticky substances of the material to be conveyed onto the circumferential surface of whose wheels. This surface can then easily be cleaned. Keeping free from engagement could be achieved in such a way that a single relative thin V-belt wheel engaged the belt about in the middle of its width, when seen in the direction of movement, by means of a V-belt vulcanized onto that surface of the belt which is averted from the side where material is to be loaded. Alternatively, a plurality of supporting wheels were rotating about the same axis which involved special requirements as to tolerances of the diameter of the wheels and their support. This, of course, could be avoided if a single wheel only was arranged in the center region. Moreover, this latter arrangement ensured that a greater part of the (lower) surface of the belt remained free which facilitated cleaning.
However, it is practically unavoidable that the broad surface of a tensioning roller engages just this inner or lower surface in the region of a tensioning arrangement. This tensioning arrangement is important, because prebiasing is necessary according to the laws of rope friction for transmitting drive torque. Moreover, the belt is easily liable to run off-track if it is not sufficiently tensioned. In such a case the deflector loops begin to migrate in axial direction of their respective wheel.
With respect to the second requirement mentioned above, the prior art tended to look for alloys which result in a maximum reliability in operation and life time. It has to be considered, however, that the ends of a metallic conveyor belt, in order to form a closed loop, have to be either riveted or welded to one another. In either case, one has to take a certain loss of elasticity into account. It should be understood that a break of the belt does not only result in costs of outage-time, but also to relative high costs of replacement, because it will be apparent from the above that the structure of such a metallic conveyor belt arrangement is relative expensive and costly.
It is, therefore, an object of the present invention to provide a metallic conveyor belt arrangement in such a way that a still higher life time can be expected.
In a first step in solving this problem, investigations have been made what the cause of breaks occurring in steel belts of the prior art might have been. Certainly, it was obvious when the belt was running that over and over new portions and surfaces contact the deflector wheels in the deflector loop of the belt, thus being subjected to flexural torque or a bending moment in addition to the normal tension which results both from prestressing the belt and from the drive moment. However, investigations showed unexpectedly that this was not the only bending moment that acted onto the belt. Just the special form of a drive which engaged the belt only along a center region resulted either in unequal stress of a plurality of supporting wheels (and therefore of the belt at the locations of engagement) and/or in too large a gap between the circumference of the supporting wheels and the (lower) engagement surface of the wheel so that the belt could easily sink in under the action of its tension.
In the case of the use of a single narrow drive wheel, the result of investigations was still more unmistakable: the tension lead to sagging of the belt at its unsupported edges, while it cambered out in the center of its width. Whenever a portion of the belt passed a deflector loop, the belt was bend in the opposite direction by the action of the weight of the conveyed material loaded on it. In addition, it was possible that the belt, closed to form a loop, could be somewhat oblique, i.e. that it was a little bit longer along one of its edges than along the other one. In this way, unequal tension over the width of the belt was additionally a result, and a higher tension was concentrated along one of the edges. This contributed necessarily to shorten the life time due to fatigue of material and aggravated the above problem.
Therefore, a solution had to be found which avoided or, at least, limited this perpetual bending to and fro in axial direction of its drive wheel. Moreover, no or smaller stress of the belt should result from the fact that it was a little bit obliquely joined to form a loop.
In a second step in the direction of solving the problem, according to a first aspect of the present invention, the recognition of the first step resulted in using at least one V-belt secured to the (lower) engagement surface, and providing a roller-shaped drive wheel which is subdivided into at least two adjacent parts secured to the drive shaft and forming a circumferential groove between them for receiving the V-belt. An axial tightening device is on the drive shaft for tightening the parts together in the direction of the shaft axis.
In this way, the tightening device will ensure that the outer surface of the V-belt is flush with the circumference of the roller-shaped drive wheel, or even may only slightly exceed this circumference, so that engagement of the V-belt and the metallic belt is ensured, while the rest of the roller can support the edges without allowing bending in axial direction of the drive wheel, i.e. the width of the metallic belt.
This solution of the problem is based on overcoming the prejudice that narrow pulley-shaped drive wheels have to be used for the metallic belt in order to minimize contamination of its engagement surface. The fact that the tensioning wheel had always a roller shape without aggravating the contamination problem substantially (because contact periods with the metallic belt are only very short) induced to find a solution of the problem where the metallic belt, although being driven by the respective V-belt, is supported by the drum""s or roller""s periphery over its overall width in a manner that bending, i.e. flexural torque, is substantially avoided.
According to a second aspect of the present invention the recognition of the first step resulted in the use of at least one tilting arrangement for enabling tilting movement of at least one of the deflector wheel or the drive shaft about a tilting axis. This tilting axis is perpendicular to the longitudinal axis of the belt and extends in a plane which intersects the slack end and the tight end of the metallic conveyor belt.
This reduces internal stress of the metallic belt by allowing compensation, e.g. in case one edge of it is slightly longer than the other one.